A waveguide orthomode transducer (OMT) is a radio frequency (RF) device often used to combine or separate orthogonally polarized signals, thus providing polarization-discrimination. OMTs also have important utility as polarization diplexers.
Unfortunately, most OMTs today are not fully satisfactory. For example, they may not be effective in preventing the generation of undesirable higher order modes, or they may not provide sufficiently high isolation between ports, or they may be difficult to manufacture and thus relatively expensive, or they may be unduly bulky and too thick for many important applications.
There are various types of OMTs, and one type based on a turnstile waveguide junction is of interest here because it can overcome some of the just noted shortcomings. Turnstile junction-based OMTs provide port isolation and suppress undesirable higher order modes, particularly across a broad bandwidth. OMTs of this type are therefore particularly used today to provide broadband continuous-wave (CW) duplexing of radio frequency (RF) energy, to generate elliptical polarizations, to transmit linear and receive cross-linear polarizations, to transmit and receive linear polarizations, and to transmit and receive circular polarizations. Such OMTs also may be used to measure the degree of ellipticity of circularly polarized waves, as main mode transducers in single or dual channel rotary joints, and as variable power dividers.
FIG. 1a-b (prior art) are depictions of a typical turnstile junction 10, as might be used in an OMT. FIG. 1a shows all of the wall structure 12 of the turnstile junction 10, with extensive ghost effect used to represent hidden lines. In contrast, FIG. 1b shows only the major structure of the turnstile junction 10, with limited use of ghost effect to represent hidden major outlines. FIG. 1b thus dispenses with the distracting detail of wall structure to facilitate showing important other features.
From FIG. 1a it can be appreciated that the turnstile junction 10 here consists, basically, of four rectangular (or ridge) waveguide ports (generically waveguide ports 14, individually waveguide ports 14a-d) that lie in a common plane and are placed symmetrically around and orthogonal to a longitudinal axis of a circular (or square) main waveguide 16. A matching element 18 (or matching elements, plural) may be provided at the base of the cavity formed by the waveguide ports 14 and the main waveguide 16 to enhance broadband operation of the turnstile junction 10 with a low reflection coefficient.
The structure depicted in FIG. 1a is merely one example, and various other shapes for the waveguide ports, the main waveguide, the cavity, and the matching elements may instead be employed in a turnstile junction. For instance, the ports can be of any transmission line type, even including planar types such as stripline.
Continuing now with FIG. 1b, the turnstile junction 10 exhibits two fundamental modes (generically modes 20, individually modes 20a-b, respectively designated Pol 1 and Pol 2 and here stylistically depicted with arrowed lines). The fundamental modes 20 can propagate in the main waveguide 16 as independent orthogonal linear polarizations, and the turnstile junction 10 splits each into equal but out-of-phase electric fields (generically e-fields 22, individually e-fields 22a-b of opposite polarity, and here also stylistically depicted with arrowed lines). The mode 20a (Pol 1) is thus split into the e-fields 22a and 22b at opposite waveguide ports 14a and 14b, but is not substantially coupled to waveguide ports 14c or 14d. Similarly, mode 20b (Pol 2) is split equally but out-of-phase into the e-fields 22a and 22b at waveguide ports 14c and 14d, but is not substantially coupled to waveguide ports 14a or 14b. 
Since the turnstile junction 10 is a reciprocal electromagnetic device, driving any two opposite waveguide ports 14 out-of-phase and with e-fields 22a and 22b of equal power will result in transferring essentially their total power to the main waveguide 16 as one of the fundamental modes 20a-b, and substantially no power will enter the other, opposite waveguide ports 14.
To make an operable OMT the four waveguide ports 14 of a turnstile junction 10 need to be connected to some other device or apparatus to provide the just discussed conditions between the respective sets of opposite waveguide ports 14. One traditional approach is to attach each set of two opposite waveguide ports 14 to an E-plane T-junction or a hybrid tee junction serving as an E-plane power divider or combiner to employ the desired equally powered but out-of-phase RF signals.
FIG. 2a-b (prior art) are depictions of a typical hybrid tee 30 (also widely termed a “hybrid junction,” “hybrid T,” and “magic T” in the art). Similar to what is done in FIG. 1b, in FIG. 2a-b ghost effect is used sparingly to represent only major hidden outlines, and the distracting detail of wall structure has been dispensed with to facilitate showing more important features. The hybrid tee 30 has one H-port 32 (also sometimes called an “H-arm”), two side-ports 33 (or “side arms,” or “symmetrical ports,” or “symmetrical arms”) and one E-port 34 (or “E-arm”).
FIG. 2a shows the hybrid tee 30 used as an E-plane power divider/combiner 36 to combine two opposed-polarity e-fields 22a and 22b into one higher power e-field 22c (or to split one high power e-field 22c into out of phase e-fields 22a and 22b having half the power each). In the E-plane power divider/combiner 36 e-fields travel via the two opposed side-ports 33, and via the E-port 34. Conversely, FIG. 2b shows the hybrid tee 30 used as an H-plane power divider/combiner 38 (which has importance discussed presently) to combine two in-phase e-fields 22b into one e-field 22d (or to split one high power e-field 22d into two in-phase, half power e-fields 22b). In the H-plane power divider/combiner 38 the e-fields 22 travel via the H-port 32 and the side-ports 33, and the E-port 34 has no active role.
FIG. 3a-c (prior art) show three different exemplary OMTs that employ turnstile junctions. FIG. 3a shows FIG. 3 of NAVARRINI & PLAMBECK, “A Turnstile Junction Waveguide Orthomode Transducer,” IEEE Transactions On Microwave Theory And Techniques, Vol. 54, No. 1, January 2006, pp. 272-77. FIG. 3b shows FIG. 1 of ARAMAKI et al., “Ultra-Thin Broadband OMT with Turnstile Junction,” IEEE MTT-S Digest, 2003, pp. 47-50; and can also be seen as FIG. 1 in U.S. Pat. App. 2005/0200430 by ARAMAKI et al., titled “Waveguide Branching Filter/Polarizer” and as FIG. 5 of U.S. Pat. No. 7,019,603 by YONEDA et al. (including Yoji ARAMAKI), titled “Waveguide Type Ortho Mode Transducer” And FIG. 3c shows FIG. 1 of U.S. Pat. No. 6,600,387 by COOK et al., titled “Multi-Port Multi-Band Transceiver Interface Assembly.” As can be appreciated by these three prior art examples, the transmission lines attached to the turnstile junction ports have to pass over each other to avoid interfering. The approaches of NAVARRINI & PLAMBECK and of CLARK et al. employ “normal” sized waveguides, and produce OMTs that are quite sizable. NAVARRINI & PLAMBECK teach fabricating their device from four machined bocks that are bolted together, thus being especially challenging to manufacture economically. The patent by CLARK et al. is specialized, resorts to waveguide pass-overs to fully utilize a turnstile junction, but can be manufactured with more conventional techniques. The approach of ARAMAKI et al. reduces the waveguide heights and uses pass-overs to minimize the total thickness. However, this excessive reduction of waveguide height compromises power handling capability, and the passing over itself increases thickness of the overall device.
In summary turnstile junction-based OMTs generally remain bulky and thick, or else require accepting undesirable performance compromises. And accordingly, what is still needed is an OMT design that provides the advantages of the turnstile junction yet permits the resulting OMT to be small and thin, yet to have high power handling capability, provides low VSWR, to provide high mode purity over a broad bandwidth, to exhibit high isolation between ports, and that is easy to manufacture.